Hometoast - just a quick post to thank you for this. I had exactly the same problem, arising from almost (but not quite) following the Ubuntu-published instructions to install Ubuntu Server 11.10 (Oneiric Ocelot) to a software RAID1 array. Exactly as you described, I had a phantom partition that would not go away, no matter what I did in fdisk or anything.
I booted into recovery mode from a memory stick, then ran:
dd if=/dev/zero of=dev/sda bs=1M dd if=/dev/zero of=dev/sdb bs=1M
And lo! The partition was gone! Currently installing Ubuntu Server on the drives in a nice tidy RAID mirror.
Aaaand this time the installer correctly installed GRUB to /dev/sda /dev/sdb (instead of last time, when it went to /dev/md0 and failed miserably). Working perfectly, huzzah!!
Thanks again, hope this post helps others in the future.